Conventional car entry systems include a remote key entry (RKE), immobilizer, and passive entry/passive start (PEPS). Some car entry systems incorporate one or more of RKE, immobilizers, and PEPS, such as the system illustrated in FIG. 1.
RKE is a coded message radiofrequency (RF) from a transmitter in a fob to a car to unlock the car doors. Typically the message is encrypted.
In a typical PEPS system, a driver may touch a car door and the vehicle will unlock (passive entry). A driver may press a start button on a dashboard to start the car (passive start). Both these actions begin an authorization processes with a fob on or very near the driver's person. A low-frequency (LF) signal from the car (typically from a PEPS transmitter) awakens the fob. The fob measures the field strength vectors of the LF signal and sends authorization data back to the vehicle. Since the fob orientation is arbitrary, it measures the field vectors in the X, Y, and Z axis. This provides three-dimensional (3D) data, which may localize the fob as either inside or outside the car. If the PEPS system determines that the fob is outside the car, it unlocks the door. If the system determines that the fob is inside the car, it starts the engine.
In a typical immobilizer system, an electronic security device is fitted to an automobile that prevents the engine from running unless the correct key (or other token) is present. The immobilizer may include an electronic device that separates the fuel system, ignition system, and engine. Alternatively, the immobilizer may include an electronic device that is connected to or otherwise interfaces with an engine management sub-system of a car control system. In some systems, a reader (sometimes called an interrogator) in a car transmits an LF interrogation signal that supplies energy through magnetic induction to a storage capacitor in a fob, and the fob initializes for LF downlink. The fob begins an authorization process with the reader, and if the authorization process is successful the immobilizer does not inhibit the engine from running.
In some cases, the same transmitter may be used for the PEPS and the immobilizer function, as with the conventional system illustrated in FIG. 1.
Both a PEPS system and immobilizer system often use one or more antennas in a car to transmit a BPLM signal to a fob. As shown in FIG. 2, BPLM involves, generally, inserting into a carrier field one or more carrier field gaps of fixed duration and setting the gap timing intervals to predetermined times for logic “0” and logic “1.” One advantage of this method is that it embeds energy transfer from vehicle to key fob into the data encoding and ensures the key fob will be supplied enough energy to process the encoded data.
Failure to detect a gap may result in an incorrectly demodulated BPLM signal. Other deficiencies and drawbacks of conventional systems may exist.